This invention relates to printing of data or information on both sides of a sheet of paper, that is, duplex printing.
Considerations of the cost of paper and convenience of handling make duplex printing desirable in copiers and non-impact printing apparatuses. In laser non-impact printers where high speed printing is an objective, it is particularly important that the inclusion of a duplex printing feature results in little or no reduction in the speed of printing. Simplicity and reliability in design for low cost production and operation are important for both copiers and laser printers.
U.S. Pat. Nos. 3,318,212; 3,536,389; 3,548,783; 3,671,118; 3,687,541; 3,697,171; 3,775,102; 3,844,653; and 3,844,654 relate to duplex copiers employing at least two separate transfer means for imaging information on both sides of a sheet of paper. The use of more than one transfer means results in higher expense and larger size.
U.S. Pat. Nos. 3,506,347; 3,615,129; 3,630,607; 3,645,615; 3,672,765; 3,856,295; 3,862,802; 3,866,904; 3,869,202; 3,947,270; 3,972,612; 4,017,181; 4,140,387; 4,158,500; and 4,174,905 relate to duplex copiers employing paper handling mechanisms for inverting the paper and presenting it to the transfer means portion of the copier a second time for duplex printing. Such mechanisms usually involve tortuous paper handling paths which result in greater complexity, and which can be inconsistent with high speed printing.
Immediate detection of paper misfeeds in copiers and particularly in very high speed laser printers employing duplex printing is important in avoiding embarrassing lack of correlation between a page of printing and pagination. It is also desirable for avoiding loss of time and waste of paper.